Seamless integration and proper co-ordination of different applications, both new and existing, among different networks are prime objectives to achieve with the implementation of the forthcoming third generation (3G) mobile system. Multimedia is one of the new applications to be part of the mobile system and H.324 based 3G-324M videotelephony is agreed to be the multimedia solution for the circuit switched (CS) part of the system. Outband indication of the multimedia call during call setup is very important in this regard to invoke the application in the peer-end, include an Interworking Function (IWF) in switching nodes and, as a whole, properly co-ordinate the whole process of a systematic call setup. Conventional PSTN (Public Switched Telephone Network) circuits lack the feature of end-to-end digital transmission and thus need a different call setup procedure in terms of Bearer Capability (BC) negotiation compared to those required for an end-to-end digital connection, available in a PLMN (Public Land Mobile Network) and an ISDN (Integrated Services Digital Network) environment. Thus, the digital end should “know” if the peer-end resides in the PSTN during call setup. But the unique numbering system for both ISDN and PSTN makes it impossible to interpret this from the dialed numbers; thus some other means is required to let the other end know about the fact during call setup.
The 3 G system is expected to work based on the existing 2G system. Thus backward compatibility with minimal changes required in existing systems is of great importance.
As mentioned, the calling/called end does not “know” from the called/calling number if the peer-end is in PSTN. Due to the different nature of the transmitting characteristics available in PSTN circuits, the mobile multimedia connection needs a different type of Bearer Service (BS) from the connection where there is PSTN involvement. Without this indication and thus fallback to the service compatible for PSTN circuit, the call would not succeed regardless of whether the PSTN-end supports the application. If this were allowed to stand, it would limit the new multimedia applications within end-to-end PLMN/ISDN circuits, keeping PSTN, still the largest telecommunications network, completely isolated.
Another important issue in this regard is that practical implementations of both ISDN and PSTN circuits are subjected to variation from one service provider to other, and national restrictions and interests also make the scenario complex as a whole.
Multimedia is a new concept to Mobile environment and thus an inter-network multimedia call between mobile and PSTN environment is also a new issue.
However, there already exist a couple of ideas to recognize the involvement of a PSTN end terminal and thus solve the problem, which are discussed briefly below.
3 GPP TS 26.112 mentions one solution each for mobile originating (MO) and mobile terminating (MT) (single numbering) cases involving PSTN. For the MO case, it recommends that the Mobile Switching Center (MSC) or switching node look for a BC Information Element (IE) in all gradual backward outband setup messages (CALL PROCEEDING, ALERTING and CONNECT which reflect the backward ISUP (Integrated Services User Part) ACM (Address Complete Message), CPG (Call Progress) and ANM (Answer Message) messages) and assume the other end to be in the PSTN, if a BC IE is not available in any of these messages. For the single numbering MT case, TS 26.112 recommends to send the SETUP message with an empty BC IE from the terminating MSC to the terminating MS. Then the terminating MS should include a series of BC IEs, including the one for 3 G324M, in the backward setup message (CALL CONFIRMED) according to its own preference. However, backward setup messages might not carry BC IE even if the other end is in PLMN or ISDN (depending on implementation). Thus, following the solution mentioned for MO case in TS.26.112, the originating MSC might end up with a completely wrong decision. This is because the PSTN specific decision is taken in the originating MSC based on a non-PSTN specific result.
On the other hand, a very simple idea also being considered for the MO case w is to initiate fallback from a default BS (UDI/RDI) to PSTN compatible BS (3.1 kHz Ext. PLMN) by the originating MSC in case it finds no UDI/RDI connection with the peer-end. Getting no UDI/RDI (Unrestricted/Restricted Digital Information) connection, however, will close the call and thus virtually require initiating a new call with new service requirement.
Both the approaches for the MO case force a wait until the end to have a decision, resulting in considerable delay in the whole call setup process.
Furthermore, for the MT single-numbering case, a partial service definition from the ISDN or PLMN end might mislead the terminating MSC, leading to a false requirement to fallback to the PSTN case. Moreover, inclusion of a series of BC IEs in the backward CALL CONFIRMED message from the terminating MS is against the existing PLMN specification.
Finally, in any case of call termination involving the PSTN, according to approaches mentioned above, the fact of PSTN involvement is finally correctly known through inband modem negotiation. Due to the possibility of misleading information, as mentioned above, the inband modem negotiation might fail, forcing an illogical service charge to the user.